For optimized encrypted MAPI, is the inner channel encrypted by default?

For optimized encrypted MAPI, is the inner channel encrypted by default?

A. Yes, always

B. No

C. Only for MS Exchange 2003 and later

D. Yes, but this is configured on the server

Reference: https://support.riverbed.com/bin/support/static/d6j3tb1i55ni1erbjp6e3fln9v/html/mhmppa9t9gq3laicvt0r91b1n7/sh_cx_9.2_ug_html/index.html#page/sh_cx_9.2_ug/setupServiceProtocolsMAPI.html